The present invention relates to novel nanoporous silica dielectric films having improved mechanical strength, and to semiconductor devices comprising these improved films. The present invention also provides improved processes for producing the same on substrates suitable for use in the production of semiconductor devices, such as integrated circuits. The nanoporous films of the invention are prepared using silicon-based starting materials and polymers, copolymers, oligomers, and/or compounds, and are prepared by a simplified process that, in one embodiment, allows for aging or gelation without heating.
As feature sizes in integrated circuits approach 0.25 xcexcm and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems. While there have been previous efforts to apply low dielectric constant materials to integrated circuits, there remains a longstanding need in the art for further improvements in processing methods and in the optimization of both the dielectric and mechanical properties of such materials used in the manufacture of integrated circuits.
Nanoporous Films
One material with a low dielectric constant are nanoporous films prepared from silica, i.e., silicon-based materials. Air has a dielectric constant of 1, and when air is introduced into a suitable silica material having a nanometer-scale pore structure, such films can be prepared with relatively low dielectric constants (xe2x80x9ckxe2x80x9d). Nanoporous silica materials are attractive because similar precursors, including organic-substituted silanes, e.g., tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d), are used for the currently employed spin-on-glasses (xe2x80x9cS.O.G.xe2x80x9d) and chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) of silica SiO2. Nanoporous silica materials are also attractive because it is possible to control the pore size, and hence the density, mechanical strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous films offer other advantages including: 1) thermal stability to 900xc2x0 C., 2) substantially small pore size, i.e., at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit, 3) as noted above, preparation from materials such as silica and TEOS that are widely used in semiconductors, 4) the ability to xe2x80x9ctunexe2x80x9d the dielectric constant of nanoporous silica over a wide range, and 5) deposition of a nanoporous film can be achieved using tools similar to those employed for conventional S.O.G. processing.
Thus, high porosity in silica materials leads to a lower dielectric constant than would otherwise be available from the same materials in nonporous form. An additional advantage, is that additional compositions and processes may be employed to produce nanoporous films while varying the relative density of the material. Other materials requirements include the need to have all pores substantially smaller than circuit feature sizes, the need to manage the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability.
Density (or the inverse, porosity) is the key parameter of nanoporous films that controls the dielectric constant of the material, and this property is readily varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to a dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the degree of porosity decreases, and vice versa. This suggests that the density range of nanoporous films must be optimally balanced between the desired range of low dielectric constant and the mechanical properties acceptable for the desired application.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate suitable for the purpose. Broadly, a precursor in the form of, e.g., a spin-on-glass composition is applied to a substrate, and then polymerized in such a way as to form a dielectric film comprising nanometer-scale voids.
When forming such nanoporous films, e.g., by spin-coating, the film coating is typically catalyzed with an acid or base catalyst and water to cause polymerization/gelation (xe2x80x9cagingxe2x80x9d) during an initial heating step.
More recently, U.S. Pat. No. 5,895,263 describes forming a nanoporous silica dielectric film on a substrate, e.g., a wafer, by applying a composition comprising decomposable polymer and organic polysilica i.e., including condensed or polymerized silicon polymer, heating the composition to further condense the polysilica, and decomposing the decomposable polymer to form a porous dielectric layer. This process, like many of the previously employed methods of forming nanoporous films on semiconductors, has the disadvantage of requiring heating for both the aging or condensing process, and for the removal of a polymer to form the nanoporous film. Furthermore, there is a disadvantage that organic polysilica, contained in a precursor solution, tends to increase in molecular weight after the solution is prepared; consequently, the viscosity of such precursor solutions increases during storage, and the thickness of films made from stored solutions will increase as the age of the solution increases. The instability of organic polysilica thus requires short shelf life, cold storage, and fine tuning of the coating parameters to achieve consistent film properties in a microelectronics/integrated circuit manufacturing process.
As mentioned supra, there is a continuing need in the microelectronics industry to provide improved materials allowing for semiconductor devices, such as integrated circuits, with increased circuit density, and increased processing speed and power. This is coupled with a continuing desire to reduce the cost in time, money and manufacturing equipment of producing such semiconductor devices. Thus, there remains this ongoing need for further improvements in both the desirable properties of nanoporous dielectric films, as well as an ongoing need for further improvements in methods for producing such nanoporous dielectric films.
In order to solve the above mentioned problems and to provide other improvements, the invention provides novel nanoporous silica dielectric films with a low dielectric constant (xe2x80x9ckxe2x80x9d), e.g., typically ranging from about 1.5 to about 3.8, as well as novel new methods of producing these dielectric films.
Broadly, the dielectric nanoporous films of the invention are prepared by a method that includes the following process steps:
(a) preparing a silicon-based, precursor composition including a porogen,
(b) coating a substrate with the silicon-based precursor to form a film,
(c) aging or condensing the film in the presence of water,
(d) heating the gelled film at a temperature and for a duration effective to remove substantially all of the porogen.
Advantageously, in the above-described process steps, the precursor composition is substantially aged or condensed in the presence of water in liquid or vapor form, without the application of external heat or exposure to external catalyst.
The artisan will appreciate that the molar ratio of water to Si can be readily determined by the desired rate of condensation and the successful production of nanoporous silica dielectric films. In particular embodiments, the molar ratio of water to Si ranges, e.g., from about 2:1 to about 0:1.
Broadly, the silicon-based precursor composition includes a monomer or prepolymer according to Formula I:
Rxxe2x80x94Sixe2x80x94Lyxe2x80x83xe2x80x83(Formula I)
wherein
x is an integer ranging from 0 to about 2, and y is an integer ranging from about 2 to about 4;
R is independently selected from the group consisting of alkyl, aryl, hydrogen and combinations thereof;
L is an electronegative moiety, such as, e.g., alkoxy, carboxy, amino, amido, halide, isocyanato and combinations thereof
The silicon-based precursor composition optionally includes one or more monomers or prepolymers of Formula I, as well as a polymer formed from the condensation of one or more different monomers or prepolymers according to Formula I. The polymer formed from Formula I has a molecular weight, for example, that ranges from about 150 to about 10,000 amu.
Useful monomers or prepolymers include, e.g., acetoxysilane, an ethoxysilane, a methoxysilane, and combinations thereof Particular monomers or prepolymers useful according to the invention also include, e.g., tetraacetoxysilane, a C1 to about C6 alkyl or aryl-triacetoxysilane, and combinations thereof. The triacetoxysilane is, for example, a methyltriacetoxysilane. Further monomers or prepolymers useful according to the invention also include, e.g., tetrakis(2,2,2-trifluoroethoxy)silane, tetrakis(trifluoroacetoxy)silane, tetraisocyanatosilane, tris(2,2,2-trifluoroethoxy)methylsilane, tris(trifluoroacetoxy)methylsilane, methyltriisocyanatosilane and combinations thereof.
Optionally, the water employed for the processes of the invention is added to the silicon-based, precursor composition prior to the application of the precursor composition to the substrate. Variations on this process include the addition of amounts of water to the silicon-based, precursor composition insufficient to fully condense or age the applied film, and completing the aging process by exposing the applied film to environmental water vapor. In one particularly convenient embodiment of the invention, no water is added to the silicon-based, precursor composition prior to application to the substrate. Instead, all of the water for aging the film is provided by environmental, e.g., atmospheric water vapor present in the controlled atmosphere of the processing facility. The atmospheric partial pressure of water vapor in the processing facility can be adjusted to range, for example, from about 5 mm Hg to about 20 mm Hg, The time required for achieving water-mediated aging depends on the materials selected, on the source of the water, e.g., mixed into the precursor or from environmental water vapor, and the desired degree of aging. The time period ranges, for example, from about 20 seconds to about 5 minutes, or more.
A useful porogen according to the invention is added to the precursor in an amount ranging from about 2 to about 20 weight percent. The porogen also has a boiling point, sublimation point or decomposition temperature ranging, e.g., from about 175xc2x0 C. to about 450xc2x0 C. The porogen also has a molecular weight ranging, e.g., from about 100 to about 10,000 amu, or more particularly, from about 100 to about 3,000 amu. In addition, the porogen is selected to be readily removed from the applied and aged film, e.g., by heating at a temperature ranging from about 175xc2x0 C. to about 300xc2x0 C., for a time period ranging from about 30 seconds to about 5 minutes to remove substantially all of the porogen.
A solvent is also optionally provided to reduce precursor viscosity and aid film spreading, as required. When a solvent is present, the silicon-based, precursor composition includes a solvent or mixture of solvents in an amount ranging, for example, from about 10% to about 90% by weight. The solvent has a boiling point ranging, for instance, from about 50 to about 175xc2x0 C. and is selected, for example, from hydrocarbons, esters, ethers, ketones, alcohols, amides and combinations thereof. However, to avoid undesirable interactions, the solvent is not an alcohol when the silicon based monomer or precursor comprises an acetoxy-functional group. To avoid cross-linkage of the solvent to the precursor, it should be noted that the solvent optionally does not include hydroxyl or amino groups. Nanoporous dielectric films prepared by the methods of the invention, as well as semiconductor devices and/or integrated circuits manufactured with such films, are also provided.